Among exim's assets are its documentation, generally responsive
and courteous (lead?) developer
Philip Hazel, and active mailing list. On the other hand,
exim is not a particular leader in regard to
security.

Several people have told me exim's configuration is "easy", in
contrast to that of, say, Zmailer, or qmail.

nullmailer
is a queuing "dumb" MTA for machines which relay to one or more
smarthosts. ... Its
design is heavily influenced by qmail, and was created by its author for
situations where a "mini-qmail" (queueless "dumb" qmail installation) was
insufficient.

It's an excellent solution for workstations in a LAN environment,
or for single Unix/Linux machines relaying to an ISP smarthost.

Correspondents have reported performance as much as three
times as great as qmail's. In any case, both qmail and Postfix
appear to be much zippier than Sendmail at high volume, although
apparently no one has reproduced such results particularly rigorously.

While I haven't yet figured out how to backup a qmail configuration
(the problem is that qmail works with inodes, so backing up as a
file-system isn't effective),
Dave Sill assures me that

Queue files are named after their i-node, but they're just normal
files. Backing up the queue is easy using any of the standard
utilities. The rub is restoring them. Luckily there are a couple
utilities that'll munge a restored queue and rename the files
properly. Pointers are on
www.qmail.org
(queue-fix, queue-rename).

Bennett Todd describes configuration for qmail, Postfix, and
smail in a
Usenet
posting.

Along with the sites mentioned at www.qmail.org [explain],
Blue Mountain Arts is a high-volume qmail user [explain].

Smail
is easy to set up, certainly compared to sendmail, from which
it borrows command-line syntax.
Some people favor it for its security and configurability.
My impression is that it doesn't scale up as well as sendmail.
I've spent little time with Smail.

Rumors have come my way that Smail maintenance is moribund.
I haven't verified these myself.

The first serious attempt I know to bring modern
security and performance concepts to bear on the
current email environment was Zmailer ...

Correspondents tell me that Zmailer is, in at least some high-volume
situations, much the best performing MTA. CommuniGate Pro also makes strong
claims about performance; so far, I've tested neither of these.
My emotional reaction in regard to the former, at least, is
skepticism, just because Zmailer is a monolith like sendmail.

Zmailer's in use at several high-profile sites. It handles
Hotmail's inbound traffic.